1. Field of the Invention
This invention relates to photodetectors and, more particularly, to photodetectors where absorption is based on an intersubband transition and gain is provided by an avalanche multiplier region.
2. Description of the Related Art
With the recent increased interest in mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) optoelectronic devices and applications, much attention has been directed to semiconductor optoelectronic devices, such as lasers, light emitting diodes (LEDs), photodetectors and the like. Particular concern has been directed to the area of detectors that operate at wavelengths between approximately 2 μm and 30 μm. Such devices are an important component in optical systems that can be used for applications including remote sensing, LADAR, detection of chemical warfare agents, intelligence surveillance and reconnaissance (ISR), enemy missile tracking and infrared countermeasures (IRCM).
Currently, high-performance photonic detectors in this wavelength range typically must be cooled to cryogenic temperatures (4-100K) to overcome deleterious effects arising due to thermionic emission. The cooling system itself can be complicated, requiring multi-stage Sterling coolers, and can comprise up to 60% of the total cost of an infrared camera based upon infrared photodetectors. These cameras have a variety of applications ranging from thermal imaging and night vision systems to effluent detection and medical diagnostics. If the operating temperature of a detector could be increased from cryogenic temperatures to temperatures achievable by the relatively inexpensive Peltier coolers (150-250K), this would lead to a significant reduction in the cost and complexity of infrared sensors and imaging systems.
State of the art MWIR and LWIR detectors are usually based on narrow bandgap mercury cadmium telluride (MCT) material, which generally offers the highest single pixel performance at a given temperature. However, non-uniformity issues associated with native defects have limited the progress of MCT-based focal plane arrays. Presently, high performance LWIR cameras used for military applications are grown on CdZnTe wafers that are expensive, can exhibit high levels of defects that subsequently degrade device performance, and are incompatible with the electronic circuitry.
One alternative to cryogenically cooled photonic detectors is bolometer-based detectors. However, this is still an emerging technology that suffers from poor performance relative to cooled detectors.
Another alternative that can be used to detect light in the >2 μm region is a quantum dot infrared photodetector (QDIP), whose operation is based on intersubband transitions of electrons. QDIPs offer many advantages. They can be operated in normal incidence. They can be based on mature GaAs-based technology. The multi-color response can be tailored from 3-30 μm. They typically have low dark current. They can also have large quantum confined Stark effect, which can be exploited to realize hyperspectral sensors. However, one of the problems facing QDIPs is their low quantum efficiency, which leads to a lower detectivity and responsivity. This, in turn, typically limits their operating temperature to about 70-80K.
In addition, the infrared wavelength region beyond approximately 2 μm is a rich area of spectroscopic research, allowing the detection of complex molecules, based on absorption arising from vibrational and rotational modes of the molecules. However, studies in this region are hampered by the absence of sufficiently sensitive detectors. Photon-counting systems are regarded as the ultimate in photon-sensing techniques from a sensitivity perspective, and have applications for sensing ultralow-level images and signals in many scientific and engineering fields stretching from microscopy and medical imaging to astronomy and astrophysics, where the photon flux is very limited. Presently, no single photon detectors are available for wavelengths beyond 2 μm.
Thus, there is a need for MWIR and longer wavelength infrared detectors that have good performance with only Peltier cooling or less. There is also a need for photon-counting and other ultra sensitive detectors at these wavelengths.